Quick questions on DC and AC motors: HSC Physics Module 6

AC motors split into two broad families. Both rely on the same idea: produce a rotating magnetic field in the stator (the stationary part) by feeding multi-phase AC into a set of coils arranged around the rotor.

The rotor is a magnet (a permanent magnet or an electromagnet fed by slip rings). It locks onto the rotating stator field and spins at exactly the same frequency (the synchronous speed, $f_{\text{rotor}} = f_{\text{supply}}$). Used in clocks, turntables and precision applications.

The rotor is a set of conducting bars short-circuited at each end (no slip rings or commutator at all). The rotating stator field sweeps past the rotor, inducing currents in the bars (Faraday's law). These currents, sitting in the rotating field, experience a magnetic force that drags the rotor in the direction of rotation (Lenz's law: the induced current opposes the change, that is, the relative motion of field past rotor).

Explain the function of the split-ring commutator in a DC motor. [2 marks]

A rectangular coil of $50$ turns, area $0.020 \text{ m}^2$, carries $3.0 \text{ A}$ in a field $B = 0.40 \text{ T}$. Calculate the maximum torque on the coil. [2 marks]

A DC motor's coil rotates at $1200 \text{ rpm}$ in a $0.50 \text{ T}$ field. (a) Explain what back EMF is and the direction it acts. (b) Sketch how the supply current changes as the motor accelerates from rest.